The present invention relates to a system for tomosynthesis of a radiopaque object or model using optical holography; more particularly, it relates to a method for producing a real image of the object using radiographic perspective views of the object from which holographic images are recorded. The real image of the object can then be viewed along any desired plane by using a light-sensitive device. That is, the real image may then be subject to tomographic scanning without destroying or detracting from the information available. The present invention is useful in the field of non-destructive testing, for example, of a fuel core for a nuclear reactor. The invention also has widespread use in the medical field for the viewing or profiling of internal organs. One of the advantages of the present invention in the medical field is that only a single set of radiographic perspectives need be taken to generate the real image of the object under study. This exposes the patient to substantially less ionizing radiation than in some prior art methods.
The following patent and publications are representative of work done in this general area: (1) Derderian, et al., U.S. Pat. No. 3,675,012 for "Cinefluorographic Holography", issued July 4, 1972; (2) Groh and Kock, MATERIALS RESEARCH BY HOLOGRAPHY, "Non-Destructive Testing", Vol. 5 October, 1972, p. 301-302; (3) Kock and Tiemens, "Tomosynthesis a Holographic Method for Variable Depth Display", OPTICS COMMUNICATIONS, Vol. 7, March, 1973, p. 260-265; (4) Grant, "Tomosynthesis; a Three-Dimensional Radiographic Imaging System", IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, Vol. 19, January, 1972, p. 20-28; and (5) Chau, "Three-Dimensional Reproduction of Shadowgrams", OPTICS COMMUNICATIONS, Vol. 4 September, 1971, p. 1-4.
In the present invention, a sequence of individual radiographs of the object are made by indexing a source of radiation along a known path. At each stop along the path a radiograph is taken on previously unexposed medium. As the radiation source progresses, an unexposed section of the medium is also advanced. Thus, there are produced a series of radiographs, each containing information concerning the object from a different perspective. These radiographs are sometimes referred to herein as "radiographic perspectives".
Each radiographic perspective is then recorded on holographic medium by exact re-tracing of the original ray paths through each radiographic perspective to duplicate the geometry under which it was originally prepared. More specifically, each radiographic perspective is illuminated by a source of coherent light through a ray backtracing lens to form an object beam. Thus, a shadowgraphic image of the radiographic perspective is projected onto a holographic medium together with a reference beam of light for the exposure. The ray back-tracing lens and object beam source are changed for each exposure to exactly duplicate the geometry under which the radiographic perspective then being holographically recorded was made. During the holographic recording, the angle of the reference beam relative to the recording medium is maintained constant. Thus, there are produced a number of separate holographically-recorded images, each corresponding to an associated radiographic perspective and containing the same optical perspective information that its associated radiographic perspective contained.
The holographically-recorded images are simultaneously illuminated with a source of coherent light from a direction exactly opposite to that used to make the recordings. Such a beam is known in the art as a "conjugate" beam. By this one means, for example, that the conjugate of a diverging beam is a converging beam whose rays exactly re-trace the rays of the diverging beam back to their point of origin and vice versa. The conjugate of a parallel beam is another parallel beam going in exactly the opposite direction relative to the orientation of the holographic recording medium.
All of the individual holographically-recorded images are then projected into space to reconstruct the real image of the original object in three dimensions. Viewing of the real image may be accomplished by any number of means, such as intercepting it with a ground-glass screen, a photographic plate, a television camera or other photosensitive device. The real image may be viewed along any arbitrary planar or non-planar surface. On such a surface, the several holographically-recorded perspectives integrate to yield a clear image of the details of the object corresponding to that portion of the surface intersecting the image while details about the object at other points are suppressed.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of one embodiment wherein identical reference numerals will refer to like parts in the various views.